Hydrogen is considered by many to be a promising energy alternative to carbon based fuels. Various technologies have been developed, and others are still being developed, regarding the production and use of hydrogen as a fuel or energy source. While many people consider hydrogen to be a desirable energy alternative to carbon based fuels, often based on the view of hydrogen being a clean energy source, various obstacles exist in creating a society that relies in substantial part on hydrogen as opposed to other forms of energy. Such obstacles generally include the ability to efficiently, safely and economically produce, transport and store hydrogen. In other words, it is desirable to make hydrogen, or other alternative energy sources, as readily available as current energy sources (such as gasoline) are in today's market.
One conventional approach to producing hydrogen includes thermochemical processes. One such process includes carrying out chemical reactions between a sulfur-iodine compound and water at high temperatures (e.g., above approximately 800° C.). Generally, the process results in the splitting of the water molecules (H2O) into hydrogen (H2) and oxygen (O2). The sulfur-iodine solution is recycled in the process and, therefore, other than hydrogen and oxygen, there are no byproducts of concern.
Another conventional approach to producing hydrogen includes the electrolysis of water. Often, without the aid of another energy source (beyond the supply of electricity), electrolysis is considered a relatively inefficient process for producing hydrogen. Indeed, the energy consumed may be more valuable than the hydrogen produced. In order to make electrolysis an economically viable process, another energy source may be incorporated into the process. For example, high-temperature electrolysis utilizes a high-temperature heat source to heat the water and effectively reduce the amount of electrical energy required to split the water molecules into hydrogen and oxygen with higher efficiencies.
In various processes where a high-temperature heat source is desired to assist with hydrogen production (in both electrolysis processes and thermochemical processes), a nuclear reactor is believed to be one appropriate source of heat. Next generation nuclear plants (NGNPs) are believed to be suitable heat sources for a variety of applications based on their capability of reaching outlet temperatures (the temperature of fluid(s) flowing out of the reactor) of approximately 900° C. to 1000° C. While such temperatures may seem particularly suitable for use in a large number of process heat applications, including the production of hydrogen, various obstacles still remain. For example, currently known materials may be limited in their ability to perform at the very high temperatures at which NGNPs may operate.
More specifically, the class of high-temperature metallic alloys contemplated for use in NGNPs (including their use in reactors, outlet piping, heat exchangers and other related components) appear to maintain their tensile strength up to about 600° C. and then begin to drop at increasing temperatures. At approximately 700° C., their drop in strength is rather noticeable and the drop in strength is rather dramatic at 800° C. (i.e., over 50% reduction as compared to that at 600° C.). Currently, no commercially available alloy is known to the inventors that may be used at temperatures above 900° C. for heat exchange applications involving anything but a negligible pressure drop across heat exchange surfaces.
Additionally, the creep rupture strength of such materials depends on the operating time at a given temperature. For example, in one high-temperature metallic alloy, at an operating time of approximately 100,000 hours (about 11 years), the rupture strength is about 240 Megapascals (MPa) at a temperature of approximately 500° C. However, the rupture strength will decrease to about 8 MPa at a temperature of about 900° C. The rupture strength also depends on the amount of time exposed to the particular temperature. Thus, at 900° C., the rupture strength of one particular high-temperature metallic alloy will increase from approximately 8 MPa to approximately 16 MPa when the operating time decreases from 100,000 hours to 10,000 hours.
Moreover, corrosion of high-temperature alloys will typically be accelerated at increased temperatures. In one example, corrosion of reactor components, including heat transfer loops and heat exchangers, may occur from impurities in the helium coolant used, for example, to cool the reactor core. Such corrosion has been shown to accelerate at temperatures in excess of approximately 800° C. unless the chemistry of the helium impurities is carefully controlled.
To underscore the issue of materials being limited in their use in high-temperature environments, it is noted that the American Society of Mechanical Engineers (ASME) codification of metallic alloys for nuclear use at 900° C. is incomplete. While a draft code was prepared for a particular alloy in 1989 that would allow for its use in specified applications at temperatures up to 982° C., the draft was not pursued to completion.
Additionally, while some ceramic materials have been proposed as substitutes for metallic alloys in high-temperature applications (based in part on their high-temperature strength remaining substantially constant from room temperature to approximately 1,000° C.), there are no ceramics that are ASME code certified for use in nuclear systems and a code case would have to be created for any specific ceramics of interest.
In light of the above issues, other approaches are needed to address the use of high-temperature heat sources in applications such as hydrogen production, such that the hydrogen production as well as other processes (i.e., those associated with the high-temperature heat source) may be carried out in an effective, efficient and safe manner.